Using Pots to blend Components

[YouAre]

Hey Guys,

Please bear with me, as I feel somewhat silly asking this question. I'm trying to understand some fundamental use of pots, mainly their functionality in blending between components.

I've seen this in a pedal:


It's my understanding that the pot blends in the Cap in parallel, thereby adding up capacitance. Is that correct?

Would another way to do this be like this?:

Lugs of pot:
1: Capacitor 1>>>Output
2: Input
3: Capacitor 2>>>Output.

Would this provide a smooth blend between Cap value 1 to Cap value 2?

Next on the block: resistors.

Say I want to have an expression pedal controlling a certain resistance, but I have NO control over the value of the pot (If I buy a cheap expression pedal that I can rewire, but can't replace the pot in). So I figure we could blend between resistances, and just use the potentiometer as a variable blender between two resistance values. The setup I had in mind would be like the capacitor setup in my previous question. This way, the value of the potentiometer in the expression pedal would only dictate the "throw" of the pedal, rather than the available resistance values. If this setup works, it would GREATLY help my ability to add expression pedals to effects.

Can you guys tell me if there's a fundamental flaw in my idea? Or am I right in my logic, and this HAS been done for years and I'm just stumbling on it now....?

Thanks for all the help again!

[Kearns892]

It's my understanding that the pot blends in the Cap in parallel, thereby adding up capacitance. Is that correct?

Yes, capacitors in parallel add, and a simple test with my multimeter and the arrangement you described using a 100k potentiometer and 100nF and 33nF  seems to work; at one extreme the meter reads approximately 33nF and just over 100nF at the other. However, there is a capacitance "sag" as the potentiometer moves from the 33nF to the 100nF wiper reaching a minimum of approximately 12nF before rapidly increasing to over 100nF (I am using an audio taper pot,  but that should not affect the apparent "sag") I wish I could offer an explanation as to what is actually occurring here, perhaps someone else with more technical knowledge would care to jump in...

The setup I had in mind would be like the capacitor setup in my previous question. This way, the value of the potentiometer in the expression pedal would only dictate the "throw" of the pedal, rather than the available resistance values.

The potentiometer would contribute to more than the "throw" of the signal. Potentiometers are essentially two variable resistors that change resistance in an inversely proportional manner. As you increase the resistance between wipers 1 and 2 the resistance between wipers 2 and 3 decreases proportionally. So at a single instance, you could picture the arrangement you propose as a split in the signal placing 4 resistors on two parallel paths, 2 from the pot, 2 fixed. In this scenario, the value of the potentiometer would contribute to the total resistance across the arrangement. Also remember resistors in parallel do not add but are equivalent to the reciprocal of the sum of the reciprocal values of the resistors. With some math I suppose you could get this arrangement to work though. It's simply not as intuitive as you hoped.

[potul]

Although it is possible to modify the value of a Variable resistor (or pot) by adding series/parallel resistors, there are some limitations to it, being the most important that fact that you will modify the response curve of the pot in a significant amount. The further you want to get from the original value, the worse. And at a certain point the pot will become completely useless.

Just do some math on paralel/serial resistors and you can find out. I have somewhere an Excel file to calculate some of these arrangements. If you're interested just ask and I see if I find it.

Potul

[YouAre]

Kearns,

I know it was silly to hope I could treat a resistor like a cap....but it was late at night, and I was hoping things would be easy for me 



Potul,

I would really appreciate that excel file. Thank you!!!

Does anyone know of any cheap and small expression pedals where you can easily replace the pot with one of our choice? Or is that asking too much also   

[Pablo1234]

actually a better way to use the expression peddle as a variable resistor is have it set as a voltage divider and send the voltage to a JFET wired as a VCR (Voltage controlled resistor) or an LDR (Light Dependant Resistor) that would be in the signal chain, The advantage to this is your peddle could be remote without having to have your signal running all over the place and you could tune the VCR/LDR resistance a heck of a lot easier than just a pot. Setting Mx and B or span and slope of the control voltage running through your foot peddle and then also wiring your VCR/LDR with parallel and or series resistors to get the proper voltage drop/voltage divider or current divider needed.

[Hides-His-Eyes]

Kearns,

I know it was silly to hope I could treat a resistor like a cap....but it was late at night, and I was hoping things would be easy for me 



Potul,

I would really appreciate that excel file. Thank you!!!

Does anyone know of any cheap and small expression pedals where you can easily replace the pot with one of our choice? Or is that asking too much also   

Bespeco volume/expression pedals take standard alpha style smooth stick pots.

[YouAre]

actually a better way to use the expression peddle as a variable resistor is have it set as a voltage divider and send the voltage to a JFET wired as a VCR (Voltage controlled resistor) or an LDR (Light Dependant Resistor) that would be in the signal chain, The advantage to this is your peddle could be remote without having to have your signal running all over the place and you could tune the VCR/LDR resistance a heck of a lot easier than just a pot. Setting Mx and B or span and slope of the control voltage running through your foot peddle and then also wiring your VCR/LDR with parallel and or series resistors to get the proper voltage drop/voltage divider or current divider needed.

I thought about that, but the reason I was looking for other methods was because I didn't like that I'd have to use tailoring resistors on the LDR. That would get us the correct upper resistance, but would mess with the lower resistance at the same time. I'm sure intuitive calculations would help though. Thank you!

Bespeco volume/expression pedals take standard alpha style smooth stick pots.

Thank you! But man, Bespeco's are kinda hard to find! There's only 1 available on ebay. And none of the big online music stores carry'em. Anyone else know any?

[Hides-His-Eyes]

Bespeco are common as muck in europe.


Try some alternative brand names for the same factory; you might have more luck with Meridian.

[potul]

Potul,

I would really appreciate that excel file. Thank you!!!

Here you have it. I tmight not be exactly what you are looking for, but can b e a good starting point.

http://sites.google.com/site/potulfx/apps/pots_taper.xls?attredirects=0&d=1

[YouAre]

Here you have it. I tmight not be exactly what you are looking for, but can b e a good starting point.

http://sites.google.com/site/potulfx/apps/pots_taper.xls?attredirects=0&d=1

Thank you very much! I've read it and I have a couple of questions, if you don't mind.

Rb is set up to be 999,999k, and that point, we can just assume it's an open circuit, right? And from this, you're essentially saying that you are using Ra to dial in the expression pedal pot to the new desired value, correct? The way I see this document is that you used Ra to turn the expression pedal into a 16-20k variable resistor between lugs 1 and 2, and the stock 100k between 2 and 3. Is this correct?


Also, This file will greatly help me in the future with the plugged in equations and the graph. Thank you so much!

[potul]

The file has 3 tabs.

First tab is using the pot as a voltage divider. Ra and Rb can be given a value. If you put a big value (like I did with the 999999) it is basically meaning it is not used (open circuit). In this case, Ra is basically "shaping" the pot response to be similar to a log taper. The voltage OUT is not linear but has a log type response.

Second tab is using the Pot as a variable resistor. In this case Rb is irrelevant and not affecting. It can be omitted. In this case you can see that the total resistance (between in and out) is Rat, and it is a combination of the pot and Ra in parallel.

Third tab is the same but in the case you want the overall R to decrease when pot is turned clockwise.

You will see that you can make the overall R smaller than the pot one, but not bigger.

If you have more questions just ask.

[YouAre]

::smacks forehead::

I hate how I ALWAYS miss the tabs at the bottom of excel pages. Potul, thanks so much for sending me this. I'm going to have to look more in depth into this later, but this will probably be a staple in my designing of circuits in the future. Thanks again!